Solution mining

In-situ Recovery (ISR), also known as in-situ leaching (ISL) or solution mining, is a mineral extraction method where ore is dissolved by a lixiviant, leached and pumped to surface. The primary minerals extracted using ISR include: copper, uranium, water-soluble salts, and lithium.

Contents

ISR Process

In-situ recovery involves a series of horizontal parallel wells that injects a solution into an orebody, while a neighboring well extracts the pregnant solution back to the surface. To detect if mining fluids are flowing outside of the mining area, monitoring wells are drilled around the saturation zone. The pregnant solution is then pumped into a storage pond before entering the processing plant. Once the desired minerals are extracted, the remaining liquid is pumped into a barren solution pond. The purpose of the secondary pond is to mix and level the acid/alkali solution before returning to leaching 1. Once desired levels of acidity are achieved, the barren solution is pumped back into the orebody for further extraction.

Acid Leaching Vs. Alkali Leaching

The two main leaching agents for ISR projects are sulfuric acid or an alkaline leach. Both solutions provide similar chemical reactions through an ion exchange in order to leach the desired minerals. The largest factor between choosing a solution is the calcium carbonate concentration in the ore body. Sulfuric acid provides a better overall recovery, but if carbonate concentration is greater than 1.5%-2% only an alkaline solution is economical 2. Other differences between acid and alkali leaching can be seen in Table 1:

CAPEX and OPEX of ISR

Capital Expenditure

Capital investments for ISR mining are significantly less than conventional open pit or underground mining. The average industry capital cost for a 1000 tpa uranium ISL mine is $148 million, including the processing plant 3. The flexibility of increasing or decreasing production also allows capital requirements to be extended across multiple years, delaying investment until the mine is generating positive cash flows.

Operating Costs

The most important parameter determining optimum operating costs is the Liquid to Solid (L:S) ratio 3. The L:S ratio is a comparison of the volume of solutions that pass through a volume of rock. A higher L:S ratio results in increased operating costs due to the higher volume of acidic solutions. Compared to conventional mining methods, ISR mines provide lower operating costs.

Applications of In-Situ Mining

Copper

Copper is the first metal to have been extracted using in-situ leaching as a mining method [4]. Historical records show that this technique was first used to recover copper in 907 AD in China however, the technology has been referenced as early as 177 BC. The in-situ copper grade is usually less than 0.5% Cu with copper carbonates being the most amenable to in-situ leaching 5. The in-situ copper extraction process is distinct from other leaching methods. In order to recover the copper, either a weak sulfuric acid or cyanide solution is used 5. The sulfuric acid solution is nearly identical to the mixture used in heap leaching operations. Processing of the pregnant solution is performed by chemical precipitation or solvent extraction electro-winning (SX/EW). When chemical precipitation is used, solvents are added to the copper solution in order to solidify copper ions to extract them from the sulfuric acid mixture. SX/EW is a significantly more complex processing method involving two separate stages. Firstly, chemicals that selectively bind to copper ions are added to the solution to improve the grade of the solution 6. The copper is extracted from the second solution using cathodes that collect pure copper ions.

In-situ leaching of copper can only be used as an extraction method in specific geological conditions. Firstly, the copper orebody must be below the natural water table 5. Secondly, the host rock must be adequately fractured to allow for the sulfuric acid to flow through and dissolve the copper reserve. A larger amount of fracturing results in a greater amount of copper being exposed to the acid allowing for a greater mining recovery 5.

Though in-situ leaching is primarily used to extract uranium, numerous mines globally are beginning to investigate the feasibility of copper in-situ leaching as technological improvements have increased the economical feasibility of such projects. Currently, there are three major projects that are attempting to use in-situ leaching to economically recover copper. All three mines are located in Arizona and include: Excelsior Mining’s Gunnison project, Copper Fox Metals’ Van Dyke project, and Taseko Mines’ Florence project. The Gunnison mine is located southeast of Tucson, Arizona. It has a 25-year mine life and will produce a total of 1.7B pounds of copper 5. The Van Dyke project is located east of Phoenix, Arizona. The average grade of the deposit is 0.25% Cu with a reserve base of 1.4B pounds of copper. Finally, the Florence project is located in Florence, Arizona. The average grade of the deposit is 0.3% Cu with a measured and indicated resource of approximately 2.8B pounds of copper 7.

Uranium

In-situ leaching (ISL) was developed in the 1970’s in the former Soviet Union and the United States. ISL was used to successfully extract uranium from sandstone type uranium deposits which could not be mined using the conventional open pit or underground mining techniques 8. This technique is now used in many countries around the world including: the USA, Kazakhstan, Uzbekistan, Australia, China, and Russia and accounted for 48% of world uranium mined in 2015. ISL is seen as the most cost effective and environmentally friendly method of mining uranium 9.

Uranium deposits that are suitable for in-situ leaching must be saturated sand or sandstone below the water table and confined above and below by a non-permeable material, such as clay. These deposits may be flat or roll-front within a permeable layer. The uranium occurs as coatings on the sand grains in the form of uraninite or coffinite 9. A cross-section illustrating a typical uranium deposit suitable for ISL is shown in figure 2:

Figure 2: Cross-section of a uranium deposit and the extraction method [9]

As the uranium is extracted from the ground, the wellfields are progressively established to follow the orebody. The wells are generally designed in hexagonal 5-spot or 7-spot patterns, shown in figure 3, which are proven to recover the uranium at a faster rate than traditional straight rows of wells 10.

Figure 3: Five and seven spot pattern of injection and extraction wells [9]

A series of monitor wells are also installed in order to monitor the flow of fluids outside of the orebody and can detect if any of the mining fluids have deviated away from the mining area. The average production life of an ISL production well pattern is generally 1 to 3 years with most of the uranium being recovered within the first 6 months of operation. Overall ore recovery ranges from 60-80% for an ISL operation 9.

The geology and groundwater conditions will determine the operating process for uranium ISL. For an orebody with significant calcium bearing minerals, such as gypsum or limestone, alkaline leaching must be used. If there is low calcium content, acid leaching must be used as it provides a higher uranium recovery than the alkaline solution 9.

The uranium complexing agents (alkaline or acid) along with an oxidant (hydrogen peroxide or oxygen) are injected into the injection wells which enter the uranium deposit, oxidizing and dissolving the in-situ uranium mineral. This process yields a uranium complex of either uranyl sulphate (UO2(SO4)34-) in an acid leach or uranyl carbonate (UO2(CO3)34-) in a alkaline leach system and can then be precipitated with an alkali 9.

The uranium-bearing solution is then pumped up the extraction wells via submersible pumps and pumped to the treatment plant where the uranium is recovered through either a resin or polymer ion exchange (IX) or liquid ion exchange (SX) system 10. The use of one system over the other depends on the quality of groundwater used. Groundwater with high concentration of ions such as chloride or nitrates, will result in low uranium loadings on the resin/polymer making SX a more feasible option. The pregnant solution is precipitated by adding ammonia, hydrogen peroxide, caustic soda or caustic magnesia then dried to produce a product containing 80-100% U3O8, depending on the temperature. The barren solution is then recharged with the complexing and oxidant agents and recycled back into the process.

Kazakhstan is the world leader in uranium ISL mining with over 19 operations in 2010 and resource estimates of 651,000 tU in 2009. Mining occurs at a depth ranging from 100-300 meters but some orebodies extend to over 800 metres below the ground surface. Uranium processing is performed using the resin/polymer method, precipitated with hydrogen peroxide, and then dried to form the final U3O8 product 9.

Australia has five uranium ISL mines with three currently in operation who produced over 8000 tonnes of U3O8 in 2015 with over 50,000 tU in resources. Deposits are generally located in buried river beds in uncemented fluvial sands ranging in depths from 100-130 metres.

Potash and Salt Mining

Solution mining can be applied to water-soluble salts (most notably potash and halite). This process entails drilling and pumping water to the deposit while pumping the resultant impregnated brine solution back to surface 11. Once the brine solution is brought to surface, evaporation is used to eliminate the water and allow for the salt to precipitate out and be treated. This can be seen in Figure 4:

Solution mining provides an opportunity to mine soft rock at depths in which conventional mining would not be feasible. Due to the decreased requirement for labour in solution mining, lower operating costs are obtained. The operational cost of potash solution mining in Canada is compared to conventional mining methods in Table 2:

Solution mining is optimal for deposits of large heights and low depths (exceeding 1000 m). The method is not suggested in areas with technical disturbances or faulting. The deposit must be relatively pure with stable geological conditions. The surrounding mineralization must be stable to avoid large amounts of surface subsidence as caving can occur from the excavation 13.

Environmental Considerations

Due to the nature of solution mining, unique risks are present while conventional mining risks are averted. The biggest concern that solution mining poses to the environment is the contamination of the ground water. Solution mining often incorporates fracking style explosions to aid the passage of the liquids underground. These explosions may create more pathways for dissolved metals, salts, natural hydrocarbons and solution mining chemicals to reach the surface or nearby water table. To prevent this, only ore bodies that are beneath an impermeable layer of clay or shale are targeted. Monitoring wells are placed above the impermeable layer to watch for any changes in the ground water content or leaks from the layer below. 14

Chemicals introduced for the solution mining processes are leaching solutions, which dissolve the desired minerals, and reducing agents that precipitate the dissolved minerals. These chemicals can have adverse effects on the local water table, bringing with them dissolved metals or minerals. 15

The United States and Canada have clean water and environmental protection acts that limit the extents of solution mining. They ensure that the ground water returns to its pre-solution mining state and limits the chemicals and dissolved solids in the ground water to be less than 500 ppm. That water must be returned to its pre-mining condition 16.

In uranium solution mining, special considerations must be made to ensure no radioactive material escapes into the local environment in any stage of the process. Although there is less contact with radioactive material via ore dust or radon release seen in conventional uranium mining, workers must still be monitored for alpha and gamma radiation to comply with OSHA guidelines 16.

The water requirements in solution mining are extremely high and can have adverse effects on the local environment if a large source of water is not readily available for use. The water can be rehabilitated after processing, however the costs and processing methods may leave water that is too contaminated to be reintroduced into the local water ways or tables. Tailing dams are used to store the contaminated water. The waste water can also be pumped back underground where the minerals were extracted however, this will require additional monitoring of the site to ensure no movement of the contaminated water to the local water table through the impermeable layer.

In salt or potash solution mining the creation of large salt caverns is common. Due to the soft nature of the rock, subsidence of the rock is common. Measures must be taken to ensure that the deposit is deep or small enough that there is no notable ground subsidence or that subsidence is permitted and allowed in that location 17.

These salt caverns can be used to store materials that do not dissolve the salt barriers. This has been used for the storage of hydrocarbon liquids and gasses, industrial waste, nuclear waste and tailings. The salt creep can cause massive loss of storage and must be compensated by the cavern’s depth, size and operating pressure. Salt falls in the cavern can cause leakage or blockage in the system. These problems have had mixed results of fixing the problem with remotely controlled submersibles.